1. Field of the Invention
The present invention relates to a fullerene derivative including perfluoropolyether (PFPE) chains and a lubricant including the fullerene derivative.
2. Description of the Related Art
As perfluoropolyether chemical compounds have large viscosity index in addition to having good heat resistance, chemical resistance and oxidation resistance, variation in fluidity (viscosity) is small over a wide temperature range from low temperature to high temperature, and show good lubricity. Further, perfluoropolyether chemical compounds have properties such as fireproof, not influencing on a high-molecular material such as rubber or plastic, low vapor pressure and low evaporation loss, low surface tension and high electric insulation, and are known to show high performance over an extremely wide range as lubricants. Thus, perfluoropolyether chemical compounds are widely used as lubricating oils for various purposes such as a vacuum pump oil, lubrication for a magnetic disk/tape or the like, a heating medium or an incoherent agent.
Meanwhile, C60, which is a fullerene, is known to be useful as a lubricant. In Non-Patent Document 1 (Bhushan et al.: Appl. Phys. Lett. 62, 3253 (1993)), it is confirmed that a coefficient of friction is lowered in a silicon substrate on which a deposited film of C60 is formed. Although Non-Patent Document 1 suggests a fullerene derivative in which a perfluoropolyether group is introduced in a fullerene, a specific chemical compound or a method of manufacturing it are not described.
Further, C60 is known to show good properties as an additive for a conventional lubricating oil. In Non-Patent Document 2 (Ginzburget et al.: Russian Journal of Applied Chemistry 75, 1330 (2002)), it is confirmed that abrasion resistance is improved by adding C60 to a lubricating oil, compared with a case when C60 is not added.
Patent Document 1 (Japanese Laid-open Patent Publication No. 2006-131874) discloses a lubricant composed of a mixture of C60, a C60 derivative including a carboxyl group, a hydroxylated fullerene or a fullerene derivative including an ester group, and a perfluoropolyether.
Although each of Patent Document 2 (Japanese Laid-open Patent Publication No. 2011-140480), Patent Document 3 (Japanese Laid-open Patent Publication No. 2013-140923) and Patent Document 4 (Japanese Laid-open Patent Publication No. 2013-170137) discloses a fullerene derivative including one perfluoropolyether chain in a molecule as an n-type semiconductor material, its purpose to be used as a lubricant is not described.
Each of Patent Document 5 (Japanese Patent No. 5600202) and Patent Document 6 (Japanese Patent No. 5600222) discloses a lubricant composed of a derivative in which one perfluoropolyether chain is provided for each cyclopropane ring that is condensed to a fullerene.